Is it correct to write "..., so..."? e.g.
You are handsome, so you are appreciated.
Is it correct to use "so" in formal writing? If not, what are the alternatives?
I try to avoid using "so" in formal / business writing. Someone suggested "therefore" as an alternative, but in some cases that may sound too strong, as it implies an indisputable logical conclusion. I don't think your example is trying to express it that strongly.
Perhaps it's helpful to put things in a different order, and use because instead:
You are appreciated, because you are handsome.
In this case, the word "therefore" would be a good alternative.
Including "and" is a touch more formal than "so" by itself.
You are handsome and so you are appreciated.
Besides the already suggested "therefore" and "because", "hence" is another less commonly used option.
You are handsome, hence you are appreciated.
A few phrases are also possible: "and for that", "and as a result"
You are handsome and for that you are appreciated.